In the field of supercharging internal combustion engines by means of turbochargers, in which the fluid admitted to the intake of an engine is compressed by a compressor driven by a turbine which extracts driving energy from the exhaust gas of the engine, there has arisen the need for multiple turbocharging systems for use with engines whose output speed varies over a working range from full rated speed down to a much lower speed which may be, for example, 50% rated speed or below. These systems require, for example, two series coupled turbochargers usually designated as low-pressure and high-pressure turbochargers. In such an arrangement, the compressor of the low-pressure turbocharger discharges into the intake of the compressor of the high-pressure turbocharger, and the exhaust gas discharge of the turbine of the high-pressure turbocharger is conducted to the inlet of the turbine of the low-pressure turbocharger.
It is obvious, of course, that the gaseous fluid flow rate increases as engine speed increases. Hence the availability of exhaust gas to drive the turbines varies with engine speed. It is desirable, however, to maintain a substantially constant level of charge pressure in the intake manifold of the engine.
In the case where there are no controls, it is known that as the through-flow of exhaust gas drops off with a decrease of engine speed and/or load, the high-pressure turbine dominates the energy extraction and thus the low-pressure turbine produces little power.
In order to increase the power output of turbines in series, it has been customary in the prior art to resort to somewhat sophisticated and expensive turbine structures, usually including complex nozzles with a plurality of variable vanes in the gas distributor and/or resorting to wasteful valve arrangements for throttling or dumping the exhaust gas flow at one or more locations in the exhaust gas flow path.